WASHINGTON — As a House-Senate conference debated a plan to balance the budget by 1991, a Nobel Prize-winning economist dismissed it Monday as ''plain Mickey Mousing'' and ''an order for disaster.''

Franco Modigliani told a congressional panel that the deficit should be reduced immediately and that a tax increase may be necessary.

He said the balanced-budget plan lacks courage and gives a false sense of security because it postpones for too long the day of reckoning.

Referring to the plan, Modigliani said, ''Many of these details are, in my view, to borrow a phrase from my grandson, plain Mickey Mousing.''

An economics professor at the Massachusetts Institute of Technology, Modigliani was awarded the 1985 Nobel Prize last week for his research on savings.

The budget plan proposes to phase out the deficit over six years by establishing an enforcement mechanism that could require spending cuts.

Developed by Sens. Phil Gramm, R-Texas, and Warren Rudman, R-N.H., the plan was endorsed by President Reagan and approved by the Republican- controlled Senate. The Democratic-controlled House approved the plan's theory but not the details.

Democrats complain that the cuts envisioned in the plan would not take effect until after the 1986 election.

If deficit ceilings were exceeded, the plan would give the president new authority to cancel, or ''sequester,'' funding that had been approved by Congress.

Modigliani's testimony to the Joint Economic Committee seemed to lend credibility to Democratic objections to the Gramm-Rudman plan.

One objection is that it gives too much authority to the president. Another is that, as it stands now, the plan would trigger the most severe cuts during a recession; the cuts would be less stringent during periods of economic good health.

Opponents say that is a backward approach to attacking the deficit.

Criticizing the timing and extent of the cuts, Modigliani said, ''It the plan tells future congressmen how to do what we don't have the guts to do now.''

Meanwhile, the House-Senate conference made little progress as it studied which federal spending programs should be subjected to Gramm-Rudman's automatic cuts.

The conference may break into task forces on individual questions. It is expected today to review scenarios of what might happen under a presidential sequester order.

In other action, the Senate debated a spending bill that would allocate $105 billion to the departments of Labor, Health and Human Services, and Education.

One would provide $1 million to study the effects of overdoses from designer drugs, synthetic equivalents of heroin and cocaine that don't fall under narcotics laws because they are derived from legal drugs.

The second Chiles amendment would compel the Social Security Administration to spend an extra $30 million on employee costs. This would thwart the Reagan administration's attempt to save money by reducing the system's bureaucracy.